1. Field of the Invention
The invention is concerned with a device for measuring the time interval separating the leading edges of two correlated pulses which have independent amplitudes and rise times, such as the output pulses of devices for detecting correlated nuclear events.
2. Description of the Prior Art
It is often necessary, especially in the field of nuclear science, to determine accurately the time interval which separates two correlated events, each of which is signalled by an output pulse from a detector. In the field of energy spectroscopy, a first detector is sensitive to an event accompanying the emission or passage of a particle, and a second detector, at a known distance from the first, is sensitive to the impact of the particle. The time interval separating the two events is a function of the speed of the particle, and therefore of its energy. For measuring the period of radioactive elements for which the period is very short, use is made of a first detector which is sensitive to the transition which generates a nuclide of the element and of a second detector which is sensitive to the break up of the nuclide. The period can then be calculated by statistically weighting the time intervals measured for a large number of nuclides.
The time interval between the pulses which mark the two events is often measured with the aid of a timing circuit which includes a time to pulse height converter and two threshold discriminators, the first of which responds to the first pulse passing through a preset amplitude threshold by starting the converter, and the second of which responds to the second pulse passing through the preset amplitude threshold by stopping the converter. A typical time to pulse height converter consists of a capacitor, a constant current source, first switching means connected between a voltage source and the capacitor, and second switching means with an input connected to the constant current source, a first output connected to the capacitor, and a second output connected to ground. The triggering of the first threshold discriminator breaks the connection between the voltage source and the capacitor, which begins to charge from the constant current source, and the triggering of the second threshold discriminator shorts the constant current to ground, interrupting the charging of the capacitor. The voltage to which the capacitor is charged, in relation to that of the voltage source, is related to the time separating the triggering of the two threshold discriminators by a particular conversion factor. This voltage can pass through a linear gate which is temporarily opened when charging is interrupted to form an output pulse with a height which represents the duration of the time interval to be measured. These output pulses can be fed to a multichannel analyser which counts the output pulses in various predetermined amplitude ranges.
Since the pulses which mark the beginning and the end of the time interval to be measured must be distinguished from the background noise from the detectors, the thresholds of the threshold discriminators cannot be set to very low values which the pulses would pass through as soon as they appear. If the pulses had very short rise times, or if the pulses at the beginning and the end of the interval were of the same shape and amplitude, the time between the triggering of the first threshold discriminator and that of the second would be equal to the duration of the time interval to be measured. In practice, different types of detector may be used to signal the beginning and the end of an interval, and the detectors may have rise times which are not negligible in comparison with the interval to be measured. Also, the amplitude and rise time of each pulse may depend on the energy of the event in question. Such measurements are subject to serious error.
With a view to reducing these errors, when the detectors and associated equipment provide pulses at the beginning and end of an interval which have the same rise time but different amplitudes, so-called `constant fraction of pulse height` threshold discriminators are used. In such discriminators, an incident pulse is fed through two paths. In one leg of the system the input pulse is attenuated to a given fraction of its original height, and in the other the pulse is inverted and delayed by an amount approximately equal to the rise time. These two signals are added together, and the threshold discriminator is set to respond to the combined signal passing through zero amplitude. This zero-crossing occurs when the extension of the rising edge of the delayed pulse is of the same amplitude as the attenuated pulse, and therefore when the pulse delayed by a known amount passes through a known fraction of the amplitude of the delayed pulse. If the rise times of the pulses at the beginning and end of the interval are equal, the moments at which the two threshold discriminators are triggered will also be delayed relative to the appearance of the pulses, the delay being equal to the delay introduced in the second signal path plus the fraction of the rise time corresponding to the passage of the pulse extension through the known fraction of the pulse amplitude.
With a view to reducing errors in measuring the time interval separating two pulses with independent amplitudes and rise times, so-called `constant delay` threshold discriminators are used. an input pulse is fed through two paths. In a first path the pulse is attenuated, generally to half its original amplitude, and in the second path the pulse is inverted and delayed by an amount less than one half the rise time. These two signals are summed and passed to a threshold discriminator which is set to trigger when the summed signal passes through zero amplitude. If it is assumed that the rising edge of the pulse is linear, if the attenuation ratio in the first signal path is 1/2, the discriminator is triggered after a delay from the appearance of the pulse which is equal to twice the delay introduced in the second signal path.
The use of constant fraction of pulse height and constant delay threshold discriminators has certain disadvantages: the two signal paths include circuits which extend the pulse rise times and which can introduce distortion, and triggering on the zero-crossing of the summed signal is subject to error due to the background noise, especially if the pulses have long rise times or low amplitudes. This amplitude is decreased by the error-correcting process itself.